A lithium secondary battery uses materials that are capable of reversibly intercalating and deintercalating lithium ions in positive and negative electrodes. A lithium secondary battery is fabricated by filling an organic electrolyte or polymer electrolyte between the positive and negative electrodes. Electrical energy is generated by an oxidation-reduction reaction during intercalation/deintercalation of lithium ions in the positive and negative electrodes.
Positive active materials include chalcogenide compounds, and examples thereof include LiCoO2, LiMn2O4, LiNiO2, LiNi1−xCoxO2 (where 0<x<1), and LiMnO2.
For negative active materials for a lithium secondary battery, lithium metal has been used, but lithium metal tends to form dendrites which can result in explosions from short circuits. Therefore, carbonaceous materials such as amorphous carbon or crystalline carbon are often used instead of lithium metal. However, the use of carbonaceous material manifests an irreversible capacity loss of 5% to 30% for the first initial several cycles. Such an irreversible capacity consumes lithium ions, and renders charge or discharge of certain of the active material particles impossible, resulting in a deterioration of energy density of the battery.
Further, such irreversible capacity problems are severely manifested on a negative active material such as Si, Sn, and the like, which have been recently studied for high capacity negative active materials.
In order to solve the above problems, U.S. Pat. No. 5,948,569 discloses that a Group 1 element of the Periodic Table may be deposited on a separator or electrode using vacuum deposition such as evaporation or sputtering to place the Group 1 element between positive and negative electrodes. However, as this method uses a deposition process, the equipment costs are very high and repair thereof is difficult. Furthermore, after the separator or electrodes are placed in a vacuum chamber, it takes a long time to develop a vacuum, making the manufacturing speed slow. Additionally, the Group 1 element, especially lithium metal, is deposited in the vacuum chamber during the deposition and should be removed periodically. However, the removal raises safety concerns because Group 1 elements have high reactivity.
U.S. Pat. No. 6,706,447 discloses a method for preparing a negative electrode by uniformly mixing a lithium metal powder and active material uniformly. However, because of the differences in density between the lithium metal and the active material, uniformity may not be obtained during slurry preparation, coating on a current collector, and the drying processes. Further, pores are generated after the lithium metal powders are dissolved during initial charge-discharge, and the pores cause a change in the electrode resulting in deterioration of both cycle life and reliability of the battery.